1. Field of the Invention
The present invention relates to a new method of conducting catalytic chemical reactions in a refinery distillation column wherein product, by-product, and unreacted feed are continuously separated. More particularly, it relates to a new method of producing high octane gasoline blending stock by catalytically reacting the dilute aromatics in certain refinery streams with olefins in other streams. The present invention is especially useful for reducing light aromatics, particular benzene, in gasoline while also converting olefins in relatively low value refinery fuel gas streams to high value liquid products.
The lead phaseout and the introduction of premium unleaded gasoline has created strong demand for increasing gasoline octane numbers. Conventional approaches such as increasing operating severity in reformers and fluid catalytic cracking units, or using octane catalysts and additives result in losses of gasoline yields. In addition, these approaches often increase the fuel gas yields in a refinery which may sometimes cause a reduction in refinery throughput and profitability.
Typical gasoline contains about 2% benzene, a chemical which has a high octane blending value, but is considered environmentally hazardous. The State of California, for example, has officially included benzene in its toxic chemicals list, and the United States Environmental Protection Agency is considering regulations to limit the amount of benzene which may be present in gasoline. It is therefore highly desirable to remove benzene from gasoline. However, physically separating benzene from gasoline has the undesirable effect of decreasing both the octane rating and volume of gasoline.
As an alternative, benzene in gasoline may be hydrogenated to a non-aromatic compound. This approach is also undesirable, however, because it requires a relatively high pressure operation and consumes hydrogen which is usually expensive in a refinery. Hydrogenation of benzene also reduces the octane rating of gasoline.
To overcome these disadvantages, it has been found that by alkylating the benzene the environmental impact is reduced, while both the octane and volume of gasoline are actually improved. The present invention addresses a novel process for the catalytic alkylation of benzene in a blending component or refinery stream during the refining process itself using an olefin-containing stream in a distillation column.
The chemical reactions involving alkylation of aromatics with olefins have been studied for a long time. For example, U.S. Pat. No. 2,860,173 discloses the use of a solid phosphoric acid (SPA) as a catalyst for the alkylation of benzene with propylene to produce cumene. U.S. Pat. No. 4,347,393 discloses the use of Friedel Crafts catalysts, especially aluminum chloride for this reaction. More recently, certain rare earth modified zeolites and Mobil's HZSM-5 zeolite catalyst have been used to carry out this reaction. Examples may be found in the Journal of Catalysis, Vol. 109, pages 212-216 (1988).
The alkylation of benzene with ethylene to produce ethylbenzene is a known commercial process. The Mobil/Badger ethylbenzene process produces high purity ethylbenzene in vapor phase with a multiple-bed reactor and a series of distillation columns. A description of the process using a dilute ethylene stream may be found in the Oil and Gas Journal , Vol. 7, pages 58-61 (1977).
It is important to distinguish that while catalytic aromatic alkylation is known, it is subject to the unexpected and unpredictable vagaries of catalytic processes. For example, in U.S. Pat. No. 3,527,823 (Jones) there is disclosed the reaction of benzene and propylene over phosphoric acid catalyst in a fixed bed upflow reactor to produce cumene. While the benzene-propylene reaction was successful, the Jones process was not applicable to the reaction of benzene and ethylene (column 13, line 36). Poor yields of ethyl benzene were obtained by Jones. However, increased ethylene purity increased the conversion of ethylene (column 13, line 10) although the yield of ethyl benzene was still not satisfactory. In another U.S. Pat. No., 3,437,705, Jones discloses the alkylation of an aromatic compound with an olefin in an aromatic to olefin mol ratio of from 2:1 to 30:1. The process is characterized by the presence of an unreacted vapor diluent, such as propane, in the reaction zone. The total alkylation effluent is passed to a flash distillation zone where the unreacted diluent is separated. The process is purportedly applicable to a variety of reactions using feedstocks containing unreactive vapor diluents.
The concept of catalytic distillation, to the extent chemical reactions and distillation are carried out in the same vessel, is known. U.S. Pat. No. 3,629,478 discloses a method for separating linear olefins from tertiary olefins by feeding a mixture of alcohol, tertiary pentenes and linear pentenes to a distillation column reactor, catalytically reacting the tertiary pentenes with the alcohol by contacting them with heterogeneous catalyst located above the feed zone, and fractionating the ether from the linear pentene in the distillation column reactor. U.S. Pat. Nos. 3,634,534 and 3,634,535 disclose a method for separating a first chemical from a mixture of chemicals using two distillation column reactors in series. In the first distillation column reactor, the first chemical undergoes a reaction to form a second chemical which is easily fractionated from the mixture of chemicals. This second chemical is then fed to the second distillation column reactor, where the reaction is reversed and the first chemical is recovered by fractionation.
U.S. Pat. Nos. 4,232,177 and 4,307,254 disclose a method for conducting chemical reactions and fractionation of a reaction mixture comprising feeding reactants to a distillation column reactor into a feed zone and concurrently contacting the reactants with a fixed bed catalytic packing to carry out both the reaction and fractionate the reaction mixture. One example is the preparation of methyl tertiary butyl ether (MTBE) in high purity from a mixed feed stream of isobutene and normal butene with a properly supported cationic ion exchange resin. U.S. Pat. No. 4,242,530 discloses a method for the separation of isobutene from a mixture comprising n-butene and isobutene by feeding a C.sub.4 stream to a distillation column reactor and contacting the stream with fixed bed acidic cation exchange resin to form disobutene which passes to the bottom of the column, said n-butene being removed overhead. U.S. Pat. No. 4,624,748 discloses a novel catalyst system for use in a distillation column reactor which includes annularly-defined spaces within the reactor.
U.S. Pat. NO. 4,849,569 (Smith) discloses a process for alkylating aromatic compounds by contacting the aromatic compound with a C.sub.2 to a C.sub.20 olefin in a distillation column reactor containing a fixed bed acidic catalyst comprising molecular sieves and cation exchange resins. The mol ratio of aromatic compounds to olefin is in the range of 2-100:1, since the greater the excess of aromatic compound the more selectivity is given to the desired product.
In spite of the art discussed, catalytic distillation reaction processes are not conventionally applied to complex hydrocarbon feedstocks and catalytic reactions thereof. It is important to distinguish that while such U.S. Pat. Nos. as 3,629,478 (Haunschild), 4,849,569 (Smith) and 4,471,154 (Franklin) disclosed the use of distillation reactors, neither suggests the use of complex refinery streams as feedstocks for such distillation reaction processes. Refinery streams are complex when they contain many different chemical components in a boiling range. Conventional distillation reaction processes are limited to reactive feed streams each of which is relatively pure, in the sense that each is composed of chemical constituents having some physical and/or chemical similarity.
A paper, "Alkylation of FCC Off-gas Olefins with Aromatics Via Catalytic Distillation", I.E. Partin, presented at the National Petroleum Refiners Association Meeting, Mar. 22, 1988, discloses a catalytic distillation process which alkylates the refiners light olefin gases such as ethylene and propylene, present in FCC and coker unit tail gas with light aromatics such as benzene and toluene, present in reformate to produce alkylated aromatics.
In the process as taught in this paper, full range reformate is charged to the lower distillation section and the total FCC off-gas stream is charged beneath the catalyst section. The solid proprietary catalyst is secured within supports which form bundles for installation in the distillation tower. As olefins and aromatics proceed into the catalyst section and react, the heavier alkylated aromatics drop out into the lower fractionation section and out the bottom of the tower with the remainder of the reformate. Light components, including light gases, proceed through the reactor and are stripped in the upper distillation section. Part of the unreacted benzene is recycled back to the tower to increase benzene conversion. Non-condensible gases go to fuel and light liquid is circulated back to the refinery gas plants or to gasoline blending.